Lung macrophages are critical for initiating the innate immune response to microbial and environmental stimuli, resolving acute inflammation, and promoting repair following injury. Persistent or dysregulated macrophage activation plays an important role in the pathogenesis of multiple lung diseases; however, studying the phenotype of the pleiotropic macrophage population in human lung disease is complicated by the invasive methods currently required to obtain sufficient quantities of cells. Development of new methodologies to non- invasively identify activated macrophages could facilitate improved understanding and treatment options for inflammatory lung diseases. A subset of macrophages at sites of inflammation expresses high levels of the folate receptor ? (FR?). Our preliminary data in mice demonstrate that FR? is not expressed in normal, quiescent lungs but is up-regulated specifically in a subset of macrophages after intratracheal or systemic treatment with E. coli lipopolysaccharide (LPS). We also identified FR? expression in lung macrophages in mouse models of bronchopulmonary dysplasia (BPD), lung fibrosis, and chronic pulmonary obstructive disease (COPD). Further, we found that lung macrophages from humans with COPD, idiopathic pulmonary fibrosis (IPF), and BPD express FR?, whereas expression in healthy human macrophages is minimal. Based on expression of this receptor, we have found that we can image activated macrophages in mouse lungs using a folate derivative conjugated with a fluorescent probe. In this proposal, we hypothesize that developing molecular imaging techniques to identify functional subsets of activated macrophages will advance understanding of inflammatory lung diseases and could lead to novel, macrophage-targeted therapies. Although our preliminary studies have employed optical imaging techniques, for applicability to humans we plan to develop positron emission tomography (PET) techniques to image activated lung macrophages. Specific aims are to: 1) identify and characterize specific cell-surface markers present on activated lung macrophages, 2) validate pre-clinical imaging approaches to visualize macrophage activation in vivo, and 3) develop PET-based strategies for imaging activated lung macrophages in vivo. Together, these studies will optimize imaging probes based on FR? expression and explore new imaging targets present on the surface of activated macrophages. These new strategies can then be applied to the study of inflammatory lung diseases in humans.